Method and system for varying gain exponentially with respect to a control signal

ABSTRACT

A method for varying gain exponentially with respect to a control signal is provided. The method includes receiving a primary control signal. A secondary control signal is generated based on the primary control signal. The secondary control signal is provided to a variable gain amplifier and is operable to exponentially vary a gain for the variable gain amplifier with respect to the primary control signal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to U.S. patent application Ser. No.______ (Attorney's Reference Number 05-LJ-058 (STMI01-05058)), titled“METHOD AND SYSTEM FOR GENERATING A TEMPERATURE-COMPENSATED CONTROLSIGNAL,” filed concurrently herewith. Patent application Ser. No. ______is assigned to the assignee of the present application. The subjectmatter disclosed in patent application Ser. No. ______ is herebyincorporated by reference into the present disclosure as if fully setforth herein.

TECHNICAL FIELD

This disclosure is generally directed to variable gain amplifiers and,more specifically, to a method and system for varying gain exponentiallywith respect to a control signal.

BACKGROUND

In wireless communication, the transmit path generally includes multiplevariable gain amplifiers (VGAs). For ease of compliance with poweradjustment specifications and for other system considerations, it isadvantageous to be able to linearly adjust the gain (in dB) of at leastone VGA in the transmit path. For some VGA designs, the exponential gainis achieved with a differential amplifier stage that provides an outputcurrent that varies exponentially in response to a differential inputcontrol voltage. The transfer function for the differential amplifier isapproximately linear-in-dB but compresses at large control voltages.This may result in problems because VGAs having non-linear-in-dBtransfer functions can cause degraded performance. For example, adistorted, or non-linear, transfer function may make it more difficultto set the transmit output power to a particular level with accuracy.

SUMMARY

This disclosure provides a method and system for varying gainexponentially with respect to a control signal.

In one aspect, a method includes receiving a primary control signal. Asecondary control signal is generated based on the primary controlsignal. The secondary control signal is provided to a variable gainamplifier and is operable to exponentially vary a gain for the variablegain amplifier with respect to the primary control signal.

In another aspect, an automatic gain control system includes a variablegain amplifier and a gain control circuit. The variable gain amplifieris operable to receive a variable gain amplifier (VGA) input signal andto generate a VGA output signal based on the VGA input signal. The gaincontrol circuit is coupled to the variable gain amplifier. The gaincontrol circuit is operable to generate a secondary control signal basedon a primary control signal and to provide the secondary control signalto the variable gain amplifier. The variable gain amplifier is furtheroperable to generate the VGA output signal based on the secondarycontrol signal.

In yet another aspect, a gain control circuit includes threetransistors, a current-controlled voltage source, a current mirror andan input current source. The first transistor has a collector that isoperable to generate an output current. The second transistor forms adifferential pair with the first transistor and has an emitter coupledto an emitter of the first transistor. The current-controlled voltagesource is coupled to a base of the first transistor. The thirdtransistor is matched to the second transistor and has an emitter thatis coupled to the current-controlled voltage source. The current mirroris coupled to a collector of the second transistor and to a collector ofthe third transistor. The input current source is coupled to the emitterof the first transistor. The gain control circuit is operable to receivea primary control signal at a base of the second transistor and a baseof the third transistor and to generate a secondary control signal atthe base of the first transistor and the base of the second transistor.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure and its features,reference is now made to the following description, taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 illustrates an automatic gain control system that is capable ofvarying gain exponentially with respect to a control signal according toone embodiment of this disclosure;

FIG. 2 illustrates a simplified circuit design of the gain controlcircuit of FIG. 1 according to one embodiment of this disclosure;

FIG. 3 illustrates a circuit diagram of the gain control circuit of FIG.1 or 2 and the variable gain amplifier of FIG. 1 according to oneembodiment of this disclosure;

FIG. 4 illustrates a method for varying gain exponentially with respectto a control signal using the gain control circuit of FIGS. 1, 2 or 3according to one embodiment of this disclosure;

FIG. 5 illustrates an automatic gain control system that is capable ofgenerating a temperature-compensated control signal according to oneembodiment of this disclosure;

FIG. 6 illustrates a block diagram of the temperature compensationcontrol circuit of FIG. 5 according to one embodiment of thisdisclosure;

FIG. 7A illustrates a circuit diagram of the thermal voltage generatorof FIG. 6 according to one embodiment of this disclosure;

FIG. 7B illustrates a circuit diagram of the thermal voltage generatorof FIG. 6 according to another embodiment of this disclosure;

FIG. 8A illustrates a circuit diagram of the current multiplier of FIG.6 according to one embodiment of this disclosure;

FIG. 8B illustrates a circuit diagram of the current multiplier of FIG.6 according to another embodiment of this disclosure;

FIG. 9 illustrates a circuit diagram of the voltage-to-current converterof FIG. 6 according to one embodiment of this disclosure;

FIG. 10 illustrates a circuit diagram of the current-to-voltageconverter of FIG. 6 according to one embodiment of this disclosure;

FIG. 11 illustrates a circuit diagram of the input circuit, thevoltage-to-current converter and the current multiplier of FIG. 6according to one embodiment of this disclosure;

FIG. 12 illustrates a method for generating a temperature-compensatedcontrol signal using the temperature compensation control circuit ofFIG. 5 or 6 according to one embodiment of this disclosure; and

FIG. 13 illustrates an automatic gain control system that is capable ofvarying gain exponentially with respect to a temperature-compensatedcontrol signal according to one embodiment of this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 13, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitably arranged variable gain circuit.

FIG. 1 illustrates an automatic gain control system 100 that is capableof varying the gain of a variable gain amplifier 105 exponentially withrespect to a control signal based on a modified version of that controlsignal received from a gain control circuit 110 according to oneparticular embodiment of this disclosure. For a particular embodiment,the automatic gain control system 100 may be implemented in a transmitpath of a communication system. However, it will be understood that theautomatic gain control system 100 may be implemented in any othersuitable system without departing from the scope of the presentdisclosure.

The gain control circuit 110 is operable to receive a primary controlsignal 135 and to generate a secondary control signal 130 based on theprimary control signal 135 in order to exponentially vary the gain ofthe variable gain amplifier 105. The variable gain amplifier 105, whichis coupled to the gain control circuit 110, is operable to receive avariable gain amplifier (VGA) input signal 120 and to generate a VGAoutput signal 125 by amplifying the VGA input signal 120 based on thesecondary control signal 130. However, the gain of the variable gainamplifier 105 varies exponentially with respect to the primary controlsignal 135 instead of with respect to the secondary control signal 130.

For the illustrated embodiment, the variable gain amplifier 105 and thegain control circuit 110 are part of an automatic gain control system100 that also comprises an amplifier 150, a detector 155, a filter 160,and a differential amplifier 165. However, it will be understood thatthe variable gain amplifier 105 and the gain control circuit 110 may beimplemented in a differently arranged automatic gain control system orin any other suitable system without departing from the scope of thepresent disclosure.

For this embodiment, the amplifier 150 is coupled to the variable gainamplifier 105 and is operable to further amplify the VGA output signal125 in order to generate a system output signal 175 that is at a higherlevel than the VGA output signal 125 for use in an application in whichthe automatic gain control system 100 is implemented. The detector 155,which is coupled to the amplifier 150, is operable to detect one or moreparameters associated with the system output signal 175, such asamplitude, carrier frequency, modulation index or the like, and togenerate a detected parameter signal 180 based on the detection.

The filter 160, which may comprise a low-pass or other suitable filter,is coupled to the detector 155 and is operable to filter out anyundesired components to generate a filtered signal 185. The differentialamplifier 165 is coupled to the filter 160 and is operable to comparethe filtered signal 185 to a reference signal 190 in order to generatethe primary control signal 135.

The gain control circuit 110, which is coupled to the differentialamplifier 165, may then generate the secondary control signal 130 basedon the primary control signal 135 and exponentially vary the gain of thevariable gain amplifier 105 with respect to the primary control signal135 using the secondary control signal 130. As described in more detailbelow in connection with FIGS. 2-4, the gain control circuit 110 isoperable to generate the secondary control signal 130 in such a manneras to maximize the ability of the variable gain amplifier 105 to varyits gain exponentially (i.e., in a linear-in-dB fashion) with respect tothe primary control signal 135.

FIG. 2 illustrates a simplified circuit design of the gain controlcircuit 110 according to one embodiment of this disclosure. Thisembodiment of the gain control circuit 110 comprises a plurality oftransistors 205, 210 and 215, a current mirror 220, a current-controlledvoltage source (CCVS) 225, and a current source 230.

The primary control signal 135, or V_(in), is applied across one of thetransistors (transistor 205) in a differential pair (transistors 205 and210). To accomplish this, transistor 215 is matched to transistor 210and is biased in the opposite direction as transistor 210 in order tocancel the voltage drop across transistor 210. In addition, the outputcurrent, identified as I_(c1), is normalized to the current source 230,which provides the input current identified as I₀.

The CCVS 225 provides a voltage, V_(c), given by the following equation:

V _(c) =V _(T) ln(I ₀ /I _(s)),

where V_(T) is the thermal voltage and the transistor exponentialcharacteristics are given by:

I _(c) =I _(s) e ^((V) _(be) /V ^(T) ⁾.

If I_(s)=I_(in), V_(be)=−V_(in), and I_(out)=I_(c), then

I_(out) =I _(in) e ^((−V) ^(in) ^(/V) ^(T) ⁾

and

A ₁=1/e ^((V) ^(in) ^(/V) ^(T) ⁾.

This relationship is provided by ensuring that V_(in) appears directlyacross transistor 205 and biasing transistor 205 such that I_(c1)=I₀when V_(in)=0.

To accomplish this, first −V_(be2) is added to the differential pair 205and 210 (as part of the CCVS 225). The current-voltage relationship maythen be written as:

I _(c1) =I _(s) e ^((−V) ^(in) ^(/V) ^(T) ⁾

and current gain is given by:

A ₁ =I _(c1) /I ₀ =e ^(((−V) ^(in) ^(−V) ^(c) ^()/V) ^(T) ⁾.

Next, −V_(c) is added to the circuit (also as part of the CCVS 225). Thecurrent-voltage relationship with −V_(c) added becomes:

I _(c1) =I _(s) e ^((V) ^(be) ^(/V) ^(T) ⁾ =I _(s) e ^(((−V) ^(in) ^(+V)^(c) ^()/V) ^(T) ⁾

and current gain becomes:

A ₁ =I _(c1) /I ₀ =e ^((−V) ^(in) ^(/V) ^(T) ⁾,

which provides the desired exponential behavior for the variable gainamplifier 105. Using this system, V_(in)=−V_(be1)+V_(c). In addition,the secondary control signal 130 is generated at the bases oftransistors 205 and 210, as indicated by the two nodes 130 a and 130 b.

FIG. 3 illustrates a circuit diagram of the variable gain amplifier 105and the gain control circuit 110 according to one embodiment of thisdisclosure. For this embodiment, the gain control circuit 110 comprisesa plurality of transistors, an operational amplifier and a currentsource.

The transistors Q1 a and Q1 b form a first current mirror, Q2 a and Q2 bform a second current mirror, and Q3 a, Q3 b and Q3 c form a thirdcurrent mirror. The transistors Q4 and Q7 are matched. For thetransistors Q3 a and Q5, V_(be) is the same, and for the transistors Q4and Q7, V_(be) is the same.

The primary control signal 135 is applied across transistors Q4 and Q7,and the secondary control signal 130 is generated from transistors Q6and Q7, as indicated at nodes 130 a and 130 b, respectively. Thesecondary control signal 130 is applied to the variable gain amplifier105 at transistors Q8 and Q9.

FIG. 4 illustrates a method for exponentially varying gain in thevariable gain amplifier 105 with respect to a primary control signal 135using the gain control circuit 110 according to one embodiment of thisdisclosure. This embodiment corresponds to the automatic gain controlsystem 100 illustrated in FIG. 1. However, it will be understood thatportions of this embodiment may be implemented in any other suitablesystem without departing from the scope of the present disclosure. Inaddition, the embodiment of FIG. 4 arbitrarily begins with the gaincontrol circuit 110; however, it will be understood that the methodcould be described beginning with any component 105, 110, 150, 155, 160or 165 of the automatic gain control system 100 and that each component105, 110, 150, 155, 160 or 165 performs its function essentiallycontinuously instead of only performing its function at a specific time,as described.

The method begins at step 405 where the gain control circuit 110receives a primary control signal 135 from the differential amplifier165. At step 410, the gain control circuit 110 generates a secondarycontrol signal 130 based on the primary control signal 135. For oneembodiment, the gain control circuit 110 generates the secondary controlsignal 130 by applying the primary control signal 135 directly acrossone transistor (such as transistor 205 or Q6) in a differential pair oftransistors (such as transistors 205 and 210 or Q6 and Q7). At step 415,the gain control circuit 110 provides the secondary control signal 130to the variable gain amplifier (VGA) 105 in order to exponentially varythe gain of the variable gain amplifier 105 with respect to the primarycontrol signal 135.

At step 420, the variable gain amplifier 105 receives the secondarycontrol signal 130 from the gain control circuit 110 and receives a VGAinput signal 120 from any suitable component coupled to the automaticgain control system 100. At step 425, the variable gain amplifier 105generates a VGA output signal 125 based on both the VGA input signal 120and the secondary control signal 130 by amplifying the VGA input signal120 with a gain that is controlled by the secondary control signal 130.

At step 430, the amplifier 150 amplifies the VGA output signal 125 togenerate a system output signal 175 for the automatic gain controlsystem 100. At step 435, the detector 155 detects one or more parametersof the system output signal 175 to generate a detected parameter signal180.

At step 440, the filter 160 filters the detected parameter signal 180 togenerate a filtered signal 185. At step 445, the differential amplifier165 compares the filtered signal 185 to a reference signal 190. At step450, the differential amplifier 165 generates the primary control signal135 based on the comparison of the filtered signal 185 to the referencesignal 190. At step 455, the differential amplifier 165 provides theprimary control signal 135 to the gain control circuit 110, and themethod returns to step 405 where the gain control circuit 110 continuesto receive the primary control signal 135.

In this way, the gain (in dB) of at least one variable gain amplifier105 in a transmit path of a communication system may be linearlyadjusted, making compliance with power adjustment specifications andother system considerations easier. The transfer function islinear-in-dB even at relatively large control voltages. This results inthe variable gain amplifier 105 having an improved performance ascompared to a variable gain amplifier with a gain that is adjusted onlyby a primary control signal from a differential amplifier. In addition,the variable gain amplifier 105 has a reduced requirement for RFcalibration.

FIG. 5 illustrates an automatic gain control system 500 that is capableof generating a temperature-compensated control signal for varying thegain of a variable gain amplifier 505 according to one embodiment ofthis disclosure. For a particular embodiment, the automatic gain controlsystem 500 may be implemented in a transmit path of a communicationsystem. However, it will be understood that the automatic gain controlsystem 500 may be implemented in any other suitable system withoutdeparting from the scope of the present disclosure.

The automatic gain control system 500 comprises a temperaturecompensation control circuit 510 that is coupled to the variable gainamplifier 505. The temperature compensation control circuit 510 isoperable to receive a constant control signal 515 and to generate atemperature-compensated control signal 520 based on the constant controlsignal 515 in order to cause the variable gain amplifier 505 to functionindependently of temperature. As used herein, a “constant controlsignal” means a control signal that is not temperature-compensated.Thus, the constant control signal 515 may be altered in order to adjustthe gain of the variable gain amplifier 505. However, the constantcontrol signal 515 is not altered to compensate for temperaturedifferences that may affect the performance of the variable gainamplifier 505.

The variable gain amplifier 505, which is coupled to the temperaturecompensation control circuit 510, is operable to receive a variable gainamplifier (VGA) input signal 530 and to generate a VGA output signal 535by amplifying the VGA input signal 530 based on thetemperature-compensated control signal 520.

For the illustrated embodiment, the variable gain amplifier 505 and thetemperature compensation control circuit 510 are part of an automaticgain control system 500 that also comprises an amplifier 550, a detector555, a filter 560, and a differential amplifier 565. However, it will beunderstood that the variable gain amplifier 505 and the temperaturecompensation control circuit 510 may be implemented in a differentlyarranged automatic gain control system or in any other suitable systemwithout departing from the scope of the present disclosure.

For this embodiment, the amplifier 550 is coupled to the variable gainamplifier 505 and is operable to further amplify the VGA output signal535 in order to generate a system output signal 575 that is at a higherlevel than the VGA output signal 535 for use in an application in whichthe automatic gain control system 500 is implemented. The detector 555,which is coupled to the amplifier 550, is operable to detect one or moreparameters associated with the system output signal 575, such asamplitude, carrier frequency, modulation index or the like, and togenerate a detected parameter signal 580 based on the detection.

The filter 560, which may comprise a low-pass or other suitable filter,is coupled to the detector 555 and is operable to filter out anyundesired components to generate a filtered signal 585. The differentialamplifier 565 is coupled to the filter 560 and is operable to comparethe filtered signal 585 to a reference signal 590 in order to generatethe constant control signal 515.

The temperature compensation control circuit 510, which is coupled tothe differential amplifier 565, may then generate thetemperature-compensated control signal 520 based on the constant controlsignal 515. As described in more detail below in connection with FIGS.6-12, the temperature compensation control circuit 510 is operable togenerate the temperature-compensated control signal 520 such that thevariable gain amplifier 505 may function independently of temperature.

FIG. 6 illustrates a block diagram of the temperature compensationcontrol circuit 510 according to one embodiment of this disclosure. Forthis embodiment, the temperature compensation control circuit 510comprises a thermal voltage generator 605, an input circuit 615, twovoltage-to-current (V-I) converters 610 and 620, a current multiplier625, and a current-to-voltage (I-V) converter 630.

The thermal voltage generator 605 is operable to generate a thermalvoltage 640. The first V-I converter 610, which is coupled to thethermal voltage generator 605, is operable to convert the thermalvoltage 640 into a thermal current 645. The input circuit 615 isoperable to receive the constant control signal 515 and to generate anadjusted constant control signal 650 based on the constant controlsignal 515 by applying an offset, if desired. In generating the adjustedconstant control signal 650, the input circuit 615 is operable to definea starting point for gain control. The second V-I converter 620, whichis coupled to the input circuit 615, is operable to convert the adjustedconstant control signal 650 into an input current 655.

The current multiplier 625, which is coupled to the V-I converters 610and 620, is operable to multiply the thermal current 645 and the inputcurrent 655 to generate an output current 660. The I-V converter 630,which is coupled to the current multiplier 625, is operable to convertthe output current 660 into the temperature-compensated control signal520.

As described in more detail below, the thermal voltage generator 605 andthe current multiplier 625 are designed to compensate for thetemperature variation in the following gain control equation:

A ₁=1/e ^((V) ^(in) ^(/V) ^(T) ⁾,

where the thermal voltage V_(T)=kT/q, by making the original V_(in) (theconstant control signal 515) a function of V_(T) (the thermal voltage640) to generate a modified V_(in) (the temperature-compensated controlsignal 520).

FIG. 7A illustrates a circuit diagram of the thermal voltage generator605 according to one embodiment of this disclosure. For this embodiment,the thermal voltage generator 605 comprises four transistors 705, 710,715 and 720. Although it would be possible to use a single differentialpair, such as transistors 705 and 710, the illustrated embodiment usestwo differential pairs. For this embodiment,

$\begin{matrix}{V_{y} = {V_{{be}\; 1} - V_{{be}\; 2} + V_{{be}\; 3} - V_{{be}\; 4}}} \\{= {V_{T}{\ln \left\lbrack {\left( {I_{c\; 1}/I_{c\; 2}} \right)\left( {I_{c\; 3}/I_{c\; 4}} \right)} \right\rbrack}}} \\{= {V_{T}{\ln \left\lbrack {\left( {I_{c\; 1}/I_{c\; 2}} \right)\left( {{mI}_{c\; 2}/{nI}_{c\; 1}} \right)} \right\rbrack}}} \\{{= {V_{T}{\ln \left( {m/n} \right)}}},}\end{matrix}$

where V_(y) is the voltage across the bases of transistors 705 and 720,as illustrated in FIG. 7A, V_(be1) is the base-emitter voltage fortransistor 705, V_(be2) is the base-emitter voltage for transistor 710,V_(be3) is the base-emitter voltage for transistor 715, and V_(be4) isthe base-emitter voltage for transistor 720.

FIG. 7B illustrates a circuit diagram of the thermal voltage generator605 according to another embodiment of this disclosure. For thisembodiment, the thermal voltage generator 605 comprises four transistors750, 755, 760 and 765 and one current source 770. The device size ratioof transistor 760 compared to transistor 765 is m:n and the currentsource 770 provides a current of (m+n)I_(c). Thus,

$\begin{matrix}{V_{y} = {V_{{be}\; 1} - V_{{be}\; 2}}} \\{{= {V_{T}{\ln \left( {m/n} \right)}}},}\end{matrix}$

where V_(y) is the voltage across the bases of transistors 750 and 755,as illustrated in FIG. 7B, V_(be1) is the base-emitter voltage fortransistor 750, and V_(be2) is the base-emitter voltage for transistor755.

FIG. 8A illustrates a circuit diagram of the current multiplier 625according to one embodiment of this disclosure. For this embodiment, thecurrent multiplier 625 comprises two transistors 805 and 810, anoperational amplifier 815, two diodes 820 and 825, and three currentsources 830, 835 and 840.

The current multiplier 625 is operable to multiply the thermal current645 (I_(T), which is provided by current source 830) and the inputcurrent 655 (I_(V), which corresponds to the collector current oftransistor 805) to generate an output current 660 (I_(out), whichcorresponds to the collector current of transistor 810) as follows:

V _(be,out) +V _(be,0) =V _(be,T) +V _(be,V)

V _(T) ln(I _(out) /I _(s))+V _(T) ln(I ₀ /I _(s))=V _(T) ln(I _(T) /I_(s))+V _(T) ln(I _(V) /I _(s))

I _(out) =I _(T) I _(V) /I ₀,

where V_(be,out) is the base-emitter voltage for transistor 810,V_(be,0) is the voltage drop across diode 825, V_(be,T) is the voltagedrop across diode 820, V_(be,V) is the base-emitter voltage fortransistor 805, and I₀ is the current provided by current source 840.

FIG. 8B illustrates a circuit diagram of the current multiplier 625according to another embodiment of this disclosure. For this embodiment,the current multiplier 625 comprises four transistors 850, 855, 860 and865, two diodes 870 and 875, and four current sources 880, 885, 890 and895. Using transistors 850 and 855 as a differential circuit in thismanner allows the current multiplier 625 to be implemented without anoperational amplifier.

This current multiplier 625 is operable to multiply the thermal current645 (I_(T), which is provided by current source 880) and the inputcurrent 655 (I_(V), which is provided by current source 885) to generatean output current 660 (I_(out), which corresponds to the collectorcurrent of transistor 855) as follows:

I _(T) ·I _(V)(I _(C) −I _(out))=I_(out)(I _(C) −I _(out))·I ₀

I _(out) =I _(T) ·I _(V) /I ₀,

where I_(C) is the current provided by current source 890 and I₀ is thecurrent provided by current source 895.

FIG. 9 illustrates a circuit diagram of the voltage-to-current converter610 according to one embodiment of this disclosure. For this embodiment,the voltage-to-current converter 610 comprises an operational amplifier905, a resistor 910, and three transistors 915, 920 and 925. Theoperational amplifier 905 comprises a positive terminal 930 and anegative terminal 935, and the resistor 910 is coupled between a node940 and the negative terminal 935 of the operational amplifier 905. Thecurrent, I_(y), through the resistor 910 is provided by the followingequation:

I _(y) =V _(T)·ln(m/n)/R _(y),

where R_(y) is the resistance provided by the resistor 910 and thevalues m and n correspond to the m and n illustrated in FIG. 7A or 7B.

The voltage across the positive terminal 930 of the operationalamplifier 905 and the node 940 corresponds to the voltage, V_(y),illustrated in FIG. 7A or 7B. Thus, for FIG. 7A, the positive terminal930 is coupled to the base of transistor 720 and the node 940 is coupledto the base of transistor 705. Similarly, for FIG. 7B, the positiveterminal 930 is coupled to the base of transistor 755 and the node 940is coupled to the base of transistor 750.

The voltage-to-current converter 610 is operable to convert the thermalvoltage 640 (V_(T), which is provided by way of the voltage V_(y)) intothe thermal current 645 (I_(T), which is provided by way of thecollector current, I_(y), of transistor 925).

FIG. 10 illustrates a circuit diagram of the current-to-voltageconverter 630 according to one embodiment of this disclosure. Thecurrent-to-voltage converter 630 comprises an operational amplifier1005, a resistor 1010, and a voltage source 1015.

The current-to-voltage converter 630 is operable to convert the outputcurrent 660 (I_(out)) generated by the current multiplier 625 into thetemperature-compensated control signal 520. For the embodiment in whichthe voltage-to-current converter 610 illustrated in FIG. 9 is used, theresistor 1010 tracks the resistor 910 in order to minimize mismatcherrors.

For this embodiment of the current-to-voltage converter 630, thetemperature-compensated control signal 520 (V_(out)) is provided by thefollowing equation:

V _(out) =I _(out) ·R _(x),

where R_(x) is the resistance provided by the resistor 1010.

FIG. 11 illustrates a circuit diagram of the input circuit 615, thevoltage-to-current converter 620, and a portion of the currentmultiplier 625 according to one embodiment of this disclosure. For thisembodiment, the input circuit 615 and the voltage-to-current converter620 comprise a transistor 1105, an operational amplifier 1110, a currentsource 1115, a resistor 1120 and a voltage source 1125. The portion ofthe current multiplier 625 illustrated comprises an operationalamplifier 1150 and two transistors 1155 and 1160.

The voltage source 1125 is operable to generate the adjusted constantcontrol signal 650 by applying an offset voltage (V_(os)) to theconstant control signal 515 (V_(v)) such that the adjusted constantcontrol signal 650 is equal to V_(os)+V_(v). The current source 1115 isoperable to provide an offset current (I_(os)). Thus, by adjusting theoffset voltage and the offset current, the input circuit 615 is operableto define a starting point for gain control for the temperaturecompensation control circuit 510.

The input circuit 615 and voltage-to-current converter 620 are operableto generate the input current (I_(v)) 655 as the collector current forthe transistor 1155 of the current multiplier 625. As described above,the current multiplier 625 is operable to multiply the input current 655by the thermal current 645 to generate the output current 660. Theoperational amplifier 1150 provides a buffer to prevent leakage currentwhen no input voltage is provided.

FIG. 12 illustrates a method for generating a temperature-compensatedcontrol signal 520 using the temperature compensation control circuit510 according to one embodiment of this disclosure. The method begins atstep 1205 where the thermal voltage generator 605 generates a thermalvoltage 640. For a particular embodiment, the thermal voltage generator605 generates the thermal voltage 640 by multiplying current ratios oftwo differential pairs of transistors, across which the total voltagedrop is V_(T)·ln(m/n). Thus, the device sizes m and n may be chosen toprovide a positive or negative voltage drop based on the correspondingapplication.

At step 1210, the voltage-to-current converter 610 converts the thermalvoltage 640 into a thermal current 645. At step 1215, the input circuit615 receives a constant control signal 515. At step 1220, the inputcircuit 615 generates an adjusted constant control signal 650 based onthe constant control signal 515. For example, the input circuit 615 mayprovide a voltage offset and/or a current offset to the constant controlsignal 515 in order to generate the adjusted constant control signal650.

At step 1225, the voltage-to-current converter 620 converts the adjustedconstant control signal 650 into an input current 655. At step 1230, thecurrent multiplier 625 multiplies the thermal current 645 and the inputcurrent 655 to generate an output current 660. For a particularembodiment, the current multiplier 625 may generate an output current660 that is equal to the thermal current 645 multiplied by the inputcurrent 655 and divided by another current, I₀. At step 1235, thecurrent-to-voltage converter 630 converts the output current 660 into atemperature-compensated control signal 520, at which point the methodcomes to an end. In this way, a temperature-compensated control signal520 may be generated that is operable to cause the variable gainamplifier 505 to function independently of temperature.

FIG. 13 illustrates an automatic gain control system 1300 that iscapable of varying the gain of a variable gain amplifier 1305exponentially with respect to a temperature-compensated control signalbased on a modified version of that control signal received from a gaincontrol circuit 1310 according to one particular embodiment of thisdisclosure. For a particular embodiment, the automatic gain controlsystem 1300 may be implemented in a transmit path of a communicationsystem. However, it will be understood that the automatic gain controlsystem 1300 may be implemented in any other suitable system withoutdeparting from the scope of the present disclosure.

In addition to the variable gain amplifier 1305 and the gain controlcircuit 1310, the automatic gain control system 500 comprises atemperature compensation control circuit 1315. The temperaturecompensation control circuit 1315 is operable to receive a constantcontrol signal 1320 and to generate a temperature-compensated controlsignal 1325 based on the constant control signal 1320 in order to causethe variable gain amplifier 1305 to function independently oftemperature.

The gain control circuit 1310, which is coupled to the temperaturecompensation control circuit 1315, is operable to receive thetemperature-compensated control signal 1325 and to generate a finalcontrol signal 1330 based on the temperature-compensated control signal1325 in order to exponentially vary the gain of the variable gainamplifier 1305. The variable gain amplifier 1305, which is coupled tothe gain control circuit 1310, is operable to receive a variable gainamplifier (VGA) input signal 1335 and to generate a VGA output signal1340 by amplifying the VGA input signal 1335 based on the final controlsignal 1330. However, the gain of the variable gain amplifier 1305varies exponentially with respect to the temperature-compensated controlsignal 1325 instead of with respect to the final control signal 1330.

For the illustrated embodiment, the variable gain amplifier 1305, thegain control circuit 1310 and the temperature compensation controlcircuit 1315 are part of an automatic gain control system 1300 that alsocomprises an amplifier 1350, a detector 1355, a filter 1360, and adifferential amplifier 1365. However, it will be understood that thevariable gain amplifier 1305, the gain control circuit 1310 and thetemperature compensation control circuit 1315 may be implemented in adifferently arranged automatic gain control system or in any othersuitable system without departing from the scope of the presentdisclosure.

For this embodiment, the amplifier 1350 is coupled to the variable gainamplifier 1305 and is operable to further amplify the VGA output signal1340 in order to generate a system output signal 1375 that is at ahigher level than the VGA output signal 1340 for use in an applicationin which the automatic gain control system 1300 is implemented. Thedetector 1355, which is coupled to the amplifier 1350, is operable todetect one or more parameters associated with the system output signal1375, such as amplitude, carrier frequency, modulation index or thelike, and to generate a detected parameter signal 1380 based on thedetection.

The filter 1360, which may comprise a low-pass or other suitable filter,is coupled to the detector 1355 and is operable to filter out anyundesired components to generate a filtered signal 1385. Thedifferential amplifier 1365 is coupled to the filter 1360 and isoperable to compare the filtered signal 1385 to a reference signal 1390in order to generate the constant control signal 1320.

The temperature compensation control circuit 1315, which is coupled tothe differential amplifier 1365, may then generate thetemperature-compensated control signal 1325 based on the constantcontrol signal 1320, and the gain control circuit 1310 may exponentiallyvary the gain of the variable gain amplifier 1305 with respect to thetemperature-compensated control signal 1325 using the final controlsignal 1330. The gain control circuit 1310 is operable to generate thefinal control signal 1330 in such a manner as to maximize the ability ofthe variable gain amplifier 1305 to vary its gain exponentially (i.e.,in a linear-in-dB fashion) with respect to the temperature-compensatedcontrol signal 1325.

It may be advantageous to set forth definitions of certain words andphrases used in this patent document. The terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation. The term “or” is inclusive, meaning and/or. The term “each”means every one of at least a subset of the identified items. Thephrases “associated with” and “associated therewith,” as well asderivatives thereof, may mean to include, be included within,interconnect with, contain, be contained within, connect to or with,couple to or with, be communicable with, cooperate with, interleave,juxtapose, be proximate to, be bound to or with, have, have a propertyof, or the like. The term “controller” means any device, system, or partthereof that controls at least one operation. A controller may beimplemented in hardware, firmware, or software, or a combination of atleast two of the same. It should be noted that the functionalityassociated with any particular controller may be centralized ordistributed, whether locally or remotely.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

1. A method for varying gain exponentially with respect to a controlsignal, comprising: receiving a primary control signal; generating asecondary control signal based on the primary control signal; andproviding the secondary control signal to a variable gain amplifier, thesecondary control signal operable to exponentially vary a gain for thevariable gain amplifier with respect to the primary control signal. 2.The method of claim 1, generating the secondary control signal based onthe primary control signal comprising applying the primary controlsignal directly across one transistor in a differential pair oftransistors.
 3. The method of claim 1, receiving the primary controlsignal comprising receiving the primary control signal from adifferential amplifier.
 4. The method of claim 1, further comprising:receiving the secondary control signal and a variable gain amplifier(VGA) input signal at the variable gain amplifier; and generating a VGAoutput signal based on the VGA input signal and the secondary controlsignal.
 5. The method of claim 4, further comprising amplifying the VGAoutput signal to generate a system output signal.
 6. The method of claim5, further comprising detecting at least one parameter associated withthe system output signal to generate a detected parameter signal, the atleast one parameter comprising at least one of amplitude, carrierfrequency and modulation index.
 7. The method of claim 6, furthercomprising filtering the detected parameter signal to generate afiltered signal.
 8. The method of claim 7, further comprising: comparingthe filtered signal to a reference signal; and generating the primarycontrol signal based on the comparing of the filtered signal to thereference signal.
 9. An automatic gain control system, comprising: avariable gain amplifier operable to receive a variable gain amplifier(VGA) input signal and to generate a VGA output signal based on the VGAinput signal; and a gain control circuit coupled to the variable gainamplifier, the gain control circuit operable to generate a secondarycontrol signal based on a primary control signal and to provide thesecondary control signal to the variable gain amplifier, the variablegain amplifier further operable to generate the VGA output signal basedon the secondary control signal.
 10. The system of claim 9, the variablegain amplifier operable to generate the VGA output signal based on thesecondary control signal by exponentially varying a gain for thevariable gain amplifier with respect to the primary control signal inresponse to the secondary control signal.
 11. The system of claim 9, thegain control circuit operable to generate the secondary control signalbased on the primary control signal by receiving the primary controlsignal directly across one transistor in a differential pair oftransistors in the gain control circuit.
 12. The system of claim 9,further comprising a differential amplifier coupled to the gain controlcircuit, the gain control circuit operable to receive the primarycontrol signal from the differential amplifier.
 13. The system of claim12, further comprising an amplifier coupled to the variable gainamplifier, the amplifier operable to receive the VGA output signal andto amplify the VGA output signal to generate a system output signal. 14.The system of claim 13, further comprising a detector coupled to theamplifier, the detector operable to detect at least one parameterassociated with the system output signal to generate a detectedparameter signal, the at least one parameter comprising at least one ofamplitude, carrier frequency and modulation index.
 15. The system ofclaim 14, further comprising a filter coupled to the detector, thefilter operable to filter the detected parameter signal to generate afiltered signal.
 16. The system of claim 15, the differential amplifierfurther operable to compare the filtered signal to a reference signaland to generate the primary control signal based on the comparing of thefiltered signal to the reference signal.
 17. A gain control circuit,comprising: a first transistor having a collector, a base and anemitter, the collector of the first transistor operable to generate anoutput current; a second transistor having a collector, a base and anemitter, the second transistor forming a differential pair with thefirst transistor, the emitter of the first transistor coupled to theemitter of the second transistor; a current-controlled voltage sourcecoupled to the base of the first transistor; a third transistor having acollector, a base and an emitter, the third transistor matched to thesecond transistor, the emitter of the third transistor coupled to thecurrent-controlled voltage source; a current mirror coupled to thecollector of the second transistor and to the collector of the thirdtransistor; an input current source coupled to the emitter of the firsttransistor; and the gain control circuit operable to receive a primarycontrol signal at the base of the second transistor and the base of thethird transistor and to generate a secondary control signal at the baseof the first transistor and the base of the second transistor.
 18. Thegain control circuit of claim 17, the current-controlled voltage sourcecomprising a fourth transistor and an operational amplifier.
 19. Thegain control circuit of claim 17, the current mirror comprising a firstcurrent mirror comprising two transistors, a second current mirrorcomprising two transistors, and a third current mirror comprising threetransistors.
 20. The gain control circuit of claim 17, the secondarycontrol signal operable to be coupled to a base of a first transistor ina variable gain amplifier and to a base of a second transistor in thevariable gain amplifier.